The Eighth
by offscreenmusings
Summary: Costia and Lexa fan fiction, from before the show starts. It takes place when Lexa becomes commander.
1. Chapter 1

POLIS - THRONE ROOM

Lexa stands in front of Costia pressing a blade to her throat. She tries to stifle her breathing, heavy with exertion and adrenaline. Everyone in the room waits in silence, waiting for Lexa to slit Costia's throat. It is their way, once chosen, a nightblood becomes Commander and the others must die.

Lexa stares at her extended arm, her hand, the beads of sweat on Costia's throat, and finally the bright blue eyes staring back at her from beneath the black hair. Staring at those blue eyes she has known since childhood, that she has loved since childhood, Lexa couldn't help but remember the first time she had set eyes on Costia.

Flashback

 _Fourteen years old, Lexa and two grounders were riding to Titus in Polis to join the other nightbloods. Riding at full speed Lexa and the two grounders nearly crashed into Costia with her one. Horses reared as they nearly collide. Spinning her horse around Lexa takes in the attire of the other group, Trikru._

 _"Ice Nation" Lexa shouts pointing in the direction they had just rode from, "keep riding"_

 _Kicking her horse into a full gallop, Lexa led the group through the forest until she found the spot she was looking for, "We fight from here" Lexa stated simply as she dismounted and drew her swords._

 _Costia swung down, hitting the ground next to Lexa._

 _"You, with the bow. Take point in the tree and take out as many as you can while they ride." Costia nodded approval at Lexa's orders._

 _Ice Nation came riding in, falling off horses as arrows flew. Swinging swords, the grounders cut out the legs of the horses as they rode by, too fast to slow at the ambush. Standing shoulder to shoulder Lexa and Costia slaughtered the Ice Nation as the fell from their horses. The group was small, easy to overpower given the element of surprise._

 _As the last Ice Nation fell, Lexa stood rigid staring at their faces, 18. Eighteen dead by her hand. She did't have the kill marks to prove it but she remembered every one of their faces._

 _Lexa finally looked at the grounders that came with her, both seemed to be fine. Only a cut on the arm which was being bandaged. Turning her head, she looked at the girl who fought next to her. Costia was standing tall, her hair hung loose over her shoulders, pieces stuck to the sweat on her brow. She wore a long sleeve shirt and slim leg pants, all as black as her hair. A tattoo crept up the side of her neck, starting somewhere under her shirt. A small gasp escaped Lexa's lips, as Costia looked her in the eyes. Lexa had never seen eyes so clear blue, almost white. They stood staring at each other._

 _Costia was the first to move, putting her sword away, she strode towards Lexa, arm extended. As she approached, Lexa noticed the height and age difference, not only was Costia a couple inches taller but probably a year or two older too. Lexa grasped the extended forearm, admiring the muscle._

 _"Costia"_

 _"Lexa"_


	2. Chapter 2

POLIS - THRONE ROOM

The air in the room was hot, filled with restless observers. Lexa could feel the stares bearing into her back. She could feel a film of sweat building on the handle of the blade. Lexa shifted her hand to keep the knife from slipping. Lexa stared at her hand, focusing on the blood under her fingernails. It was dark, black, the blood of the seven she killed to be closer to ascending to Commander. A demonstration of strength, that the Commander's spirit was right to choose her. Lexa knew this kill would be the hardest, staring into the eyes of her lover and slitting her throat.

Costia never took her eyes off Lexa's face. She wanted nothing more than to comfort her, to remind Lexa that this was her duty to become Commander, that their people needed her. Costia leaned forward into the knife, feeling the sear as a drip of blood rolled down her throat. Lexa needed to be reminded Costia would forgive her. That this needed to be done. Lexa's green eyes snapped onto hers, shifting slightly. Costia knew Lexa understood but her eyes pleaded for another answer.

"Do it" her lips barely moved but Lexa was watching them.

Flashback

 _Costia and Lexa had been training together for weeks. Sparring together under the direction of the Commander and Titus. It was weeks of Costia beating Lexa. Recently, Lexa had stepped up her game, winning match after match against the other nightbloods._

 _Titus had paired everyone up to spar with swords, Costia and Lexa took up a fighting stance against each other. Costia's eyes danced with amusement as she lunged forward hitting Lexa with the broadside of her sword. Her reach was longer then Lexa's but Lexa was faster on her feet. Ego bruised from the smack, Lexa went at Costia head on, swords clashing, each leveraging for power. Lexa pushed against Costia with everything she had, forcing Costia back a step. Lexa side stepped right as fast as she could, rolling off Costia's shoulder. The sudden disappearance of weight caused Costia to lose her balance, stumbling forward. Lexa pounced, kicking out Costia's feet. Pulling a dagger from her hilt she pulled on Costia's hair forcing her head back Lexa placed the knife on her throat. Lexa looked to Titus to call the match. At a slight nod and Lexa dropped her hand. Refusing to let exhaustion get to her, she stepped around Costia pulling her to her feet. Holding forearms, Costia laughed at Lexa's serious face. With her free hand she pushed stray hair behind Lexa's ear, her palm lingering on Lexa's cheek._

 _"_ _Well done Heda" Costia whispered._

 _"_ _None of us are Heda yet" Costia leaned in, placing her forehead against Lexa's their breath mingling together._

 _"_ _But you are the only here worthy of such a title" placing a kiss on Lexa's cheek she sauntered out of the clearing._

 _Lexa jogged down the hill after Costia, blood pounding in her ears._

 _"_ _Costia!"_

 _"_ _Yes Lexa?" Costia turned her brows furrowed in confusion as Lexa stopped her with a hand on the shoulder. Stepping closer Lexa pressed her lips against Costia's. Costia placed her hand on the back of Lexa's neck, pulling Lexa closer, angling her head to deepen their kiss. Hands glided over each other's bodies, learning every curve and listening to every breath taken, every moan and quiver. Costia was the first to break the kiss for air, backing them up against a tree as Lexa's lips latched onto her neck. Groaning, Costia leaned her head against the tree enjoying the sensations coursing through her body, Lexa's leg pushed firmly between hers. Lexa gasped as Costia's cold hands settled on the sensitive skin under the hem of her shirt._

 _"_ _We should stop" Costia gasps "We don't want to do this here"_

 _Lexa pulled her head away from Costia, taking in the messy black hair and swollen lips. She moved her head slightly in acknowledgement. Keeping eye contact Lexa stepped back letting the cool air rush between their bodies._

 _"_ _What would Titus say if he saw us?" Costia and Lexa looked up the hill with amusement._

 _"_ _Love is weakness" Costia deepened her voice, imitating Titus._

Flashback

 _Laying between Costia's legs, Lexa propped her chin on her arm crossed over Costia's chest. Costia had one hand on Lexa's back, her other hand stroking Lexa's hair. Smiling, Lexa placed a kiss to Costia's abdomen._

 _"_ _Was that good?" Lexa looked at Costia waiting for approval._

 _"_ _Are you sure you had never done that before?" Costia joked, as if this was their first time. Lexa rewarded her with a grin as she crept lower placing kisses on the inside of Costia's thighs._

 _Lexa lay on her stomach next to Costia._

 _"_ _We need to talk about something Heda" Lexa groaned as she turned her head to Costia._

 _"_ _I wish you would not call me that"_

 _"_ _Lexa I'm serious. We need to talk about this. The Commander has died and the conclave is going to be happening soon. Titus has told us how the conclave goes, when the Commander's spirit chooses, the rest of us will die"_

 _"_ _Costia…"_

 _"_ _No Lexa! I need you to know, when the time comes, you have to kill me. You have too. Our people will not accept you as the Commander's choice if you cannot." Costia propped herself up on one hand raising herself over Lexa. Costia placed her hand in Lexa's, gripping it._

 _"_ _When the times comes you have to kill me Lexa" Tears started to roll down Costia's face._

 _"_ _You need to know it is okay. I need you to not hurt over it. Don't let killing me kill you"_

 _Silence fell between the two of them as they stared at each other._

 _"_ _I will do what is right when the time comes" Lexa squeezed Costia's hand._


	3. Chapter 3

POLIS - THRONE ROOM

Streaks of blood ran down Costia's neck as she pushed herself further into the blade of Lexa's knife.

"Do it" Costia's lips barely moved but the message was clear.

"Ai gonplei nou ste odon, Lexa"

"No" Lexa whispered, lowering the knife from Costia's throat. Murmurs spread through the throne room as ambassadors looked at one another for explanation.

"Lexa" Titus stepped forward, only to be cut off as Lexa's voice rang off the blocks in the room.

"The Commander's spirit has chosen me! And yet our traditions deem it necessary that I show the strength in the Commander's choice."

Lexa stepped past Costia to stand in front of the throne.

"Should it not be the Commander's spirit alone that chooses? Is that not our way?"

Head high, Lexa stared at the room daring someone to step forward, to question her.

"The conclave is over, let the ascension begin." Lexa looked towards Titus' voice and nodded.

Titus motioned as a seat and tools came forward and Costia was escorted from the room. Lexa stepped forward, taking a seat with her back to the ambassadors. All Lexa could hear was her shirt tearing. Her breath took a sharp intake as cool air hit the sweat on her back. Lexa sat up as a cloth was dragged along her spine cleaning the surface.

Lexa stared forward, baring her teeth as Titus began the tattoo along her spine. Lexa knew the importance of the ritual, but she could not focus on Titus' words. Lexa went in and out of listening to Titus and counting her breathes. _Eight. Did he say eight?_

"Seven" Silence.

"I only killed seven Titus" Lexa's voice came out strong from behind clenched teeth.

POLIS - COMMANDER'S CHAMBERS

Costia sat next to Lexa laying on the bed, her shirt open to let the fresh tattoo breath. She couldn't help but stare, counting the seven circles, over and over again.

"Why did you do it Lexa?" Costia refused to meet her eyes as Lexa turned to face her.

"Because Titus is wrong, love is not weakness" Lexa placed her hand on Costia's thigh.

"I am my best when I am with you, and our people deserve my best. I could not let you die"

Costia's eyes slowly made their way to Lexa's, hair falling over one eye.

"I love you too Heda" Lexa's lips curved into a small smile.

Costia let her eyes slowly rake over Lexa's body letting desire flood her face.

"Now, are you too sore to move or what?"

Lexa's face broke into a smile, leaning in to capture Costia's lips.

"Oh, I think I can manage"


End file.
